Abstract

The neural basis of temporal processing is unclear. We addressed this important issue by performing two experiments in which repetitive transcranial magnetic stimulation (rTMS) was administered in different sessions to the left or right supramarginal gyrus (SMG) or vertex; in both tasks, two visual stimuli were presented serially and subjects were asked to judge if the second stimulus was longer than the first (standard) stimulus. rTMS was presented on 50% of trials. Consistent with a previous literature demonstrating the effect of auditory clicks on temporal judgment, rTMS was associated with a tendency to perceive the paired visual stimulus as longer in all conditions. Crucially, rTMS to the right SMG was associated with a significantly greater subjective prolongation of the associated visual stimulus in both experiments. These findings demonstrate that the right SMG is an important element of the neural system underlying temporal processing and, as discussed, have implications for neural and cognitive models of temporal perception and attention.

A schematic display of SET. A pacemaker and accumulator mechanism constitutes the “internal clock” stage, in which pacemaker pulses are passed into an accumulator. Working memory is recruited when the accumulated duration must be retained for a short period of time, whereas reference memory is necessary if a previously experienced duration must be retrieved. A decision stage mechanism serves as comparator between durations of different lengths. Attention influences temporal perception via a gate mechanism, which allows pacemaker pulses to enter the accumulator.

Baseline data from a representative subject performing a temporal discrimination task utilizing the PEST algorithm. Plotted points represent the range of comparison durations tested, and the probability of responding longer for those durations. The size of the plotted points represents the number of trials tested with that comparison duration.

Task design for Experiments 1 and 2. Subjects viewed a red square for either 600 msec or their upper threshold (UT) duration, taken from their baseline performance. After a gap of 4000 msec, a second red square was presented for either duration again. Subjects were required to judge whether the second red square persisted for shorter or longer than the first. Stimulation was administered in Experiment 1 at the onset of the first visual stimulus, and at the onset of the second visual stimulus in Experiment 2.

Changes in the proportion of trials on which subjects judged the second stimulus to be longer after receiving rTMS to the right supramarginal gyrus (RSMG) or left supramarginal gyrus (LSMG), as compared to vertex stimulation in (A) Experiment 1 and (B) Experiment 2. Plotted values were obtained for each subject by subtracting the average proportion of trials on which subjects judged the second stimulus to be longer for both sites from the vertex. Asterisks represent significant differences versus vertex stimulation in repeated measures ANOVAs.